Rare earth-doped core optical fiber and manufacturing method thereof

ABSTRACT

A rare earth-doped core optical fiber of the present invention includes a core comprising a silica glass containing at least aluminum and ytterbium, and a clad provided around the core and comprising a silica glass having a lower refraction index than that of the core, wherein the core has an aluminum concentration of 2% by mass or more, and ytterbium is doped into the core at such a concentration that the absorption band which appears around a wavelength of 976 nm in the absorption band by ytterbium contained in the core shows a peak absorption coefficient of 800 dB/m or less.

TECHNICAL FIELD

The present invention relates to a rare earth-doped core optical fiber,and to a manufacturing method thereof. The rare earth-doped core opticalfiber according to the present invention is used as a fiber for opticalamplification of an optical fiber laser, an optical amplifier, etc. andis particularly suitable for the constitution of an optical fiber laser.

This application is a continuation application based on a PCT PatentApplication No. PCT/JP2006/321385, filed Oct. 26, 2006, whose priorityis claimed on Japanese Patent Application No. 2005-311002, filed on Oct.26, 2005. The contents of both the PCT Application and the JapaneseApplication are incorporated herein by reference.

BACKGROUND ART

Recently, it has been reported that a single-mode optical fiber laser oroptical amplifier, which employs an optical fiber doped with a rareearth element such as neodymium (Nd), erbium (Er), praseodymium (Pr),and ytterbium (Yb), as a laser active medium (hereinafter referred to asa rare earth-doped optical fiber,) has many possible applications inwide fields such as optical sensing or optical communication, and theirapplicability has been expected. One example of applications thereof isan Yb-doped core optical fiber laser employing an optical fiber in whicha core is doped with Yb (which is hereinafter referred to as a Yb-dopedcore optical fiber), which is examined for the use in marker, repairing,soldering, cutting/drilling, welding for various materials or the like,and then commercialized. Conventionally, the laser used in such materialprocessing applications has been mainly a YAG laser, but recently therequirements for the processing performance have become more stringent,and as a result, the needs of laser performance have increased. Forexample,

1. a smaller spot size is required in order to achieve high precisionprocessing;

2. a higher output power is required; and

3. a reduction in down time for maintenance, etc. of a laser (such asMTBF, and MTBM) is required.

For these requirements, the Yb-doped core optical fiber laser ischaracterized in that it has

1. a spot size in a μm-order;

2. a several W through several kW output power; and

3. an expected life time of 30,000 or more, and the Yb-doped coreoptical fiber laser has a greater advantage when compared to aconventional YAG laser.

As the rare earth-doped core optical fiber, there is generally known anoptical fiber obtained by using a rare earth-doped glass, as describedin Patent Documents 1 and 2. The rare earth-doped glass is doped with arare earth element, aluminum, and fluorine in a host glass comprising aSiO₂-based composition, and the rare earth-doped core optical fiberincludes the glass as a core. Accordingly, the core part is doped with arare earth element, aluminum, and fluorine.

If a SiO₂ glass or a GeO₂—SiO₂-based glass, used for common opticalfibers, is doped with about 0.1% by mass or more of a rare earthelement, there occurs a problem of a so-called concentration quenching.This is a phenomenon where rare earth ions are aggregated (clustered)with each other in the glass, whereby the energy of excited electrons islikely to be lost in a non-radial process, leading to a reduction offluorescence life time or of fluorescence efficiency. Patent Document 1describes that by doping both of the rare earth element and Al, a highconcentration of the rare earth element can be doped without causingdeterioration of the light emitting characteristics, and even with alower interaction length with the pump light, a sufficient amplificationgain is attained, thereby making it possible to realize a small-sizedlaser or optical amplifier.

Patent Document 2 describes a method for manufacturing a rareearth-doped core optical fiber, and in particular a rare earth-dopedglass. In this method, a preform of a silica porous glass having an openpore connected therewith is immersed in a solution containing a rareearth ion and an aluminum ion, and the rare earth element and thealuminum are impregnated in the preform. Thereafter, a drying process iscarried out, in which the preform is dried, the chloride of the rareearth element and the aluminum are deposited in the pores of thepreform, and the deposited chloride is oxidized and stabilized. Then,the preform after the drying process is sintered for vitrification.Further, at a time between the completion of the drying process and thesintering process, the preform is subject to heat treatment under anatmosphere containing fluorine to dope the fluorine.

A rare earth-doped core optical fiber is obtained by synthesizing glass,as a clad portion, around the obtained rare earth-doped glass to obtaina glass preform for manufacturing of an optical fiber; and thenfiber-drawing the preform. Herein, in order to obtain an optical fiberthat is used for an Yb-doped core optical fiber laser, ytterbium (Yb)may be used as a rare earth element in the manufacturing process for therare earth-doped glass.

An example of other methods for manufacturing an Yb-doped core opticalfiber is a combination of a MCVD process and a solution process, asdescribed in Non-Patent Document 1. In this method, SiCl₄, GeCl₄, O₂gases, etc. are firstly flowed through a silica glass tube which is tobe served as a clad glass, and a heat source such as an oxyhydrogenburner disposed outside the silica glass tube is used to oxidize SiCl₄and GeCl₄ and to produce SiO₂ and GeO₂ glass soots, which are thendeposited inside the silica glass tube. At this time, the temperatureduring deposition is kept to not give a completely transparent glass,thus obtaining a glass in a porous state. Next, a solution containing Ybions is introduced into the inside of the silica glass tube having theprepared porous glass layer therein, and penetrated into the porousportion. After the sufficient penetration time with the solution, thesolution is withdrawn from the silica glass tube, and the tube isdehydrated to remove water under a chlorine atmosphere. Then, the porousportion is made transparent, and core solidification is performed toprepare a preform for a Yb-doped core optical fiber. If necessary, theYb-doped core optical fiber is obtained by synthesizing a glass, as aclad portion, around the prepared preform, thereby giving a transparentglass preform for preparation of an optical fiber; and thenfiber-drawing the preform. Further, the obtained optical fiber can beused to constitute an Yb-doped core optical fiber laser.

FIG. 1 is a configuration diagram showing one example of the Yb-dopedcore optical fiber laser, in which the Yb-doped core optical fiber laserhas a constitution comprising a Yb-doped core optical fiber 1, LD 2 as apump light source connected to input the pump light from one end of thefiber, and optical fiber gratings 3 and 4 connected to both ends of theYb-doped core optical fiber 1.

-   [Patent Document 1] Japanese Unexamined patent Application, First    Publication No. 11-314935-   [Patent Document 2] Japanese Unexamined Patent Application, First    Publication No. 3-265537-   [Non-Patent Document 1] Edited by Shoichi SUDO, Erbium-doped optical    fiber amplifier, The Optronics Co., Ltd.-   [Non-Patent Document 2] Laser Focus World Japan 2005. 8, p.p. 51-53,    published by Co., Ltd. E-express-   [Non-Patent Document 3] Z. Burshtein, et. al., “Impurity Local    Phonon Nonradiative Quenching of Yb3+ Fluorescence in    Ytterbium-Doped Silicate Glasses”, IEEE Journal of Quantum    Electronics, vol. 36, No. 8, Exit 2000, pp. 1000-1007

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present inventors have observed that when a conventionalmanufacturing method was used to prepare a Yb-doped core optical fiberto constitute the Yb-doped core optical fiber laser as shown in FIG. 1to try a laser oscillation, the output power of the light at a laseroscillation wavelength of 1060 nm decreases over time, and as a result,the laser oscillation stops. Furthermore, the present inventors havealso observed that this phenomenon also occurs in a commerciallyavailable Yb-doped core optical fiber from a manufacturer as an opticalfiber for an optical fiber laser. For this reason, it has been provedthat the conventional Yb-doped core optical fiber cannot endure over along period of time. Non-Patent Document 2 shows that such a decrease inthe output power of the laser oscillation light occurs due to aphenomenon called as ‘photodarkening’. Furthermore, it is believed thatthe above-described phenomenon is a phenomenon in which the output powerof the laser oscillation light is decreased, due to loss by the power ofthe pump light and the laser oscillation light caused by photodarkening.

The photodarkening phenomenon is one that clearly differs from theabove-described concentration quenching. The concentration quenching isa phenomenon in which rare earth ions are aggregated (clustered) witheach other in the glass, whereby the energy of excited electrons islikely to be lost in a non-radial process. Since there is usually nochange in the aggregation state of the rare earth ions during the laseroscillation, the laser oscillation, even carried out over a long periodof time, does not cause the change in the degree of concentrationquenching and decrease in the output power of the laser oscillation overtime. Patent Documents 1 and 2 in prior art may solve the concentrationquenching on an optical fiber obtained by employing a rare earth-dopedglass, but they cannot solve the problems on the decrease in the outputpower of the laser oscillation caused from a photodarkening phenomenon.

Under these circumstances, the present invention has been made, and anobject of which is to provide a rare earth-doped core optical fiber thatcan be used to prepare an optical fiber laser capable of maintaining asufficient output power of laser oscillation, even carried out over along period of time, and a manufacturing method thereof.

Means to Solve the Problems

In order to accomplish the object, the present invention provides a rareearth-doped core optical fiber, which includes a core comprising asilica glass containing at least aluminum and ytterbium, and a cladprovided around the core and comprising a silica glass having a lowerrefraction index than that of the core, wherein aluminum and ytterbiumare doped into the core such that a loss increase by photodarkening,T_(PD), satisfies the following inequality (A):T _(PD)≧10^({−0.655*(D) ^(Al) ^()−4.304*exp{−0.00343*(A) ^(Yb)^()}+1.274})  (A)

[in inequality (A), TPD represents an allowable loss increase byphotodarkening at a wavelength of 810 nm (unit: dB), D_(Al) representsthe concentration of aluminum contained in the core (unit: % by mass),and A_(Yb) represents the peak absorption coefficient of the absorptionband which appears around a wavelength of 976 nm in the absorption bandby ytterbium contained in the core (unit: dB/m)].

Furthermore, the present invention provides a rare earth-doped coreoptical fiber, which comprises a core comprising a silica glasscontaining aluminum and ytterbium, and a clad provided around the coreand comprising a silica glass having a lower refraction index than thatof the core, wherein the core has an aluminum concentration of 2% bymass or more, and ytterbium is doped into the core at such aconcentration that the absorption band of ytterbium doped into the core,which appears around a wavelength of 976 nm, shows a peak absorptioncoefficient of 800 dB/m or less.

In the rare earth-doped core optical fiber of the present invention, itis preferable that the core also contains fluorine.

In the rare earth-doped core optical fiber of the present invention, itis preferable that a polymer layer having a lower refraction index thanthat of the clad is provided on the periphery of the clad.

In the rare earth-doped core optical fiber, it is preferable that theclad is composed of an inner clad positioned on the exterior of thecore, and an outer clad positioned outside the inner clad, and that therefractive index n1 of the core, the refractive index n2 of the innerclad, the refractive index n3 of the outer clad, and the refractiveindex n4 of the polymer layer satisfy the relationship of n1>n2>n3>n4.

In the rare earth-doped core optical fiber of the present invention, airholes may be present in a part of the clad glass.

Furthermore, the present invention provides a manufacturing method of arare earth-doped core optical fiber. The method includes a depositionstep which includes introducing raw material gases composed of variouskinds of a halide gas and an oxygen gas from a first cross-section ofthe glass tube having silica as a main component into a hollow portionof the glass tube, heating the glass tube by a heating means, subjectingthe halide gas to oxidization to form a soot-like exit, depositing thesoot-like exit on the inner surface of the glass tube, and sinteringdeposited soot-like exit to deposit the porous glass layer; a dopingstep which includes doping an additive into the porous glass layer ofthe inner surface of the glass tube after the deposition step; atransparentization step which includes heating the glass pipe to subjectthe porous glass layer to transparent glass after the doping step; acore solidification step which includes collapsing a hollow portion ofthe glass tube for core solidification to form a preform after thetransparentization step; and a fiber-drawing step which includesfiber-drawing the optical fiber preform including the preform to obtaina rare earth-doped core optical fiber after the core solidificationstep, wherein the halide gas contains at least SiCl₄ and AlCl₃, theadditive contains at least a rare earth element, and in either or bothof the deposition step and the transparentization step, a fluoride gasis introduced from a first cross-section of the glass tube to a hollowportion of the glass tube.

In the manufacturing method of the present invention, it is preferablethat the rare earth element used as the additive at least containsytterbium.

In the manufacturing method of the present invention, it is preferablethat the core of the obtained rare earth-doped core optical fiber has analuminum concentration of 2% by mass or more, and ytterbium is dopedinto the core at such a concentration that the absorption band ofytterbium doped into the core which appears around a wavelength of 976nm shows a peak absorption coefficient of 800 dB/m or less.

In the manufacturing method of the present invention, it is preferablethat the method further comprises a step for forming a polymer layerhaving a lower refraction index than that of the clad on the peripheryof the clad of the optical fiber in the fiber-drawing step.

In the manufacturing method of the present invention, it is preferablethat the clad of the obtained rare earth-doped core optical fiber iscomposed of an inner clad positioned on the exterior of the core, and anouter clad positioned outside the inner clad, and the refractive indexn1 of the core, the refractive index n2 of the inner clad, therefractive index n3 of the outer clad, and the refractive index n4 ofthe polymer layer satisfy the relationship of n1>n2>n3>n4.

Advantages of the Invention

As for the rare earth-doped core optical fiber of the present invention,when the rare earth-doped core optical fiber of the present invention isused for an optical fiber laser having ytterbium as a laser activemedium, the laser oscillation, even carried out over a long period oftime, only slightly decreases the output power of the light at a laseroscillation wavelength, and enables to manufacture an optical fiberlaser capable of maintaining a sufficient output power of laseroscillation even with use over a long period of time.

By using the manufacturing method of the rare earth-doped core opticalfiber of the present invention, a rare earth-doped core optical fiberthat is capable of manufacturing an optical fiber laser capable ofmaintaining a sufficient output power of laser oscillation even with useover a long period of time can be efficiently manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing one example of the optical fiberlaser.

FIG. 2A is a schematic cross-sectional view showing a first example of afirst embodiment of the rare earth-doped core optical fiber of thepresent invention.

FIG. 2B is a schematic cross-sectional view showing a third example of afirst embodiment of the rare earth-doped core optical fiber of thepresent invention.

FIG. 3 is a graph showing the absorption spectrum by Yb of the Yb-dopedcore optical fiber of the present invention.

FIG. 4A is a schematic view showing one example of the manufacturingmethod of the rare earth-doped core optical fiber according to thepresent invention, and is a cross-sectional view showing the depositionstep.

FIG. 4B is a schematic view showing one example of the manufacturingmethod of the rare earth-doped core optical fiber according to thepresent invention, and is a cross-sectional view showing the dopingstep.

FIG. 5A is a schematic view showing one example of the manufacturingmethod of the rare earth-doped core optical fiber according to thepresent invention, and is a cross-sectional view showing the dryingprocess.

FIG. 5B is a schematic view showing one example of the manufacturingmethod of the rare earth-doped core optical fiber according to thepresent invention, and is a cross-sectional view showing the dehydrationprocess.

FIG. 5C is a schematic view showing one example of the manufacturingmethod of the rare earth-doped core optical fiber according to thepresent invention, and is a cross-sectional view showing thetransparentization step.

FIG. 5D is a schematic view showing one example of the manufacturingmethod of the rare earth-doped core optical fiber according to thepresent invention, and is a cross-sectional view showing the coresolidification step.

FIG. 6 is a schematic cross-sectional view showing a second embodimentof the rare earth-doped core optical fiber of the present invention.

FIG. 7 is a schematic cross-sectional view showing a third embodiment ofthe rare earth-doped core optical fiber of the present invention.

FIG. 8 is a block diagram showing the measurement sequence for measuringthe loss increase by photodarkening used in the Examples.

FIG. 9A is a graph showing the results of the loss increase byphotodarkening as measured in Example 1.

FIG. 9B is a graph showing the results of the loss increase byphotodarkening as measured in Example 1.

FIG. 10 is a graph showing the results of the loss increase byphotodarkening as measured in Example 2.

FIG. 11 is a graph showing the results of the loss increase byphotodarkening as measured in Example 3.

FIG. 12 is a graph showing the results of the loss increase byphotodarkening as measured in Example 4.

FIG. 13 is a graph showing the relationship between the loss increase byphotodarkening and the Al concentration at an absorption coefficient of800 dB/m.

REFERENCE NUMERALS

1: Yb-doped core optical fiber, 2: Pump light source, 3 and 4: Opticalfiber gratings, 10B to 10E: Rare earth-doped core optical fibers, 11B to11E: Cores, 12B to 12D: Clads, 13: Polymer layer, 14: Inner clad, 15:Outer clad, 20: Silica glass tube, 21: Porous glass layer, 22:Oxyhydrogen burner, 23: Aqueous solution, 24: Plug, 25: Transparentglass layer, 26: Core portion, 27: Clad glass layer, and 28: Preform.

BEST MODE FOR CARRYING OUT THE INVENTION

According to Patent Documents 1 and 2, a rare earth-doped glass having arare earth element, aluminum, and fluorine doped in a host glass havingSiO₂-based composition, and a manufacturing method thereof aredisclosed, wherein ytterbium (Yb) is used as a rare earth element, andfurther the Yb-doped glass is used in the core portion to make anYb-doped core optical fiber, which can be also applied in prior art.However, Patent Documents 1 and 2 have a detailed description thaterbium (Er) is chosen as a rare earth element, but have no descriptionof ytterbium being chosen as a rare earth element. Furthermore, thetechnology as described in Patent Documents 1 and 2 is a means forsolving a problem on concentration quenching of a rare earth element,and thus it cannot be applied to solve the problem of the decrease inthe output power of the laser oscillation light over time by using anYb-doped core optical fiber (photodarkening problem) in prior art. Thatis, it is known that since the energy level that relates in the laseroscillation of the ytterbium ion (Yb³⁺) in the Yb-doped core opticalfiber is only in two kinds of states, that is, a ²F_(7/2) ground stateand a ²F_(5/2) excited state, very little concentration quenchingoccurs. Further, Non-Patent Document 3 describes that the ytterbiumconcentration upon generation of concentration quenching in the glasshaving neither aluminum nor fluorine doped thereinto is 5×10²⁰ cm⁻³. TheYb-doped core optical fiber used in the optical fiber laser generallyhas such an ytterbium concentration that the absorption band whichappears around a wavelength of 976 nm shows the peak absorptioncoefficient in a range of from 100 to 2000 dB/m. The ytterbiumconcentration, as determined through calculation using these values,0.11×10²⁰ cm⁻³ to 2.2×10²⁰ cm ³, which is smaller than that upongeneration of concentration quenching as described in Non-PatentDocument 3. Therefore, it is believed that aluminum is not needed toinhibit the concentration quenching of ytterbium.

On the other hand, a method of doping aluminum into the Yb-doped coreoptical fiber, as described later, can be a means for solving theproblem on the decrease in the output power of the laser oscillationlight, but the amount of aluminum doped is even more than that requiredto inhibit concentration quenching. For example, the concentrationquenching was not observed in the Yb-doped core optical fiber, in whichthe core has a fluorine concentration of 0.6% by mass and an aluminumconcentration of 0.1% by mass, and ytterbium is doped at a concentrationsuch that the absorption band which appears around a wavelength of 976nm in the absorption band by ytterbium contained in the core shows thepeak absorption coefficient of 1000 dB/m, but remarkable increase in thephotodarkening loss was observed in the fiber. Further, the fluorescencelife time was measured on several other Yb-doped core optical fibers, inwhich the core has a fluorine concentration of 0.6% by mass and analuminum concentration of 0.1% by mass, and the absorption coefficientis in a range of from 200 dB/m to 1900 dB/m. The results are shown inTable 1.

TABLE 1 Fluorescence life span of Yb-doped core optical fiber Yb lightabsorption rate (dB/m) Fluorescence life span (ms) 200 0.8 600 0.8 10000.8 1400 0.8 1900 0.8

Regardless of the absorption coefficient, the fluorescence life time isa constant value, and accordingly, even if the absorption coefficient isin a range of from 200 dB/m to 1900 dB/m, the concentration quenchingdoes not occur.

Patent Documents 1 and 2 as prior arts do not describe appropriateconcentrations of ytterbium, aluminum, and fluorine, and thus it isdifficult to solve a problem on the decrease in the output power of thelaser oscillation light even using the Yb-doped core optical fiber inprior art.

On the other hand, in order to solve a problem on the decrease in theoutput power of the laser oscillation light, the rare earth-doped coreoptical fiber of the present invention is a rare earth-doped coreoptical fiber, which comprises a core comprising a silica glasscontaining aluminum and ytterbium, and a clad provided around the coreand comprising a silica glass having a lower refraction index than thatof the core, wherein a concentration of aluminum contained in the core,and the peak absorption coefficient of the absorption band which appearsaround a wavelength of 976 nm in the absorption band by ytterbiumcontained in the core, are adjusted, respectively, so as to obtain anallowable loss increase by photodarkening.

First Example of First Embodiment

A first example of the first embodiment of the rare earth-doped coreoptical fiber according to the present invention is described withreference to FIG. 2A. The rare earth-doped core optical fiber 10B in thepresent example is composed of a core 11B doped with a rare earthelement and a clad 12B surrounding the core, having a lower refractiveindex than the core.

The rare earth-doped core optical fiber 10B shown in FIG. 2A has a core11B comprising a silica glass containing aluminum (Al) and an ytterbium(Yb) that is a rare earth element, and a clad 12B comprising a silica(SiO₂) glass provided around the core. Furthermore, the core has an Alconcentration of 2% by mass or more. In addition, Yb is contained in thecore at a concentration such that the absorption band which appearsaround a wavelength of 976 nm shows the peak absorption coefficient 800dB/m or less in the absorption by Yb contained in the core. FIG. 3 showsone example of the absorption spectrum by Yb of the rare earth-dopedcore optical fiber according to the present invention.

If an optical fiber laser is constituted by using a rare earth-dopedcore optical fiber having Yb doped into the core, an optical fiber laserproviding an output power of a light as a laser oscillation wavelengthof 1060 nm is obtained. However, an optical fiber laser using aconventional Yb-doped core optical fiber has a phenomenon that theoutput power of a light as a laser oscillation wavelength of 1060 nm isdecreased over time, and as a result, laser oscillation stops.

On the other hand, for the optical fiber laser constituted by using therare earth-doped core optical fiber of the present invention, thedecrease rate of the output power of the light at a laser oscillationwavelength of 1060 nm can be significantly reduced even when laseroscillation is carried out over a long period of time. As the core has ahigher Al concentration, the optical fiber laser has a lower decreaserate in the output power of the laser oscillation. Further, as the corehas a higher Yb concentration, the optical fiber laser has a higherdecrease rate in the output power of laser oscillation. As a result, bymaking the Al concentration of the core and the absorption coefficientby Yb of the rare earth-doped core optical fiber to suitable rangedescribed in the present invention, the decrease rate in the outputpower in the optical fiber laser can be significantly reduced.

Second Example of First Embodiment

The second example of the present embodiment of the rare earth-dopedcore optical fiber is described by way of specific examples. The rareearth-doped core optical fiber of the present example has substantiallythe same basic structure as that of the rare earth-doped core opticalfiber shown in FIG. 2A, but it is a rare earth-doped core optical fiberwhich has the core 11B comprising a silica glass containing aluminum(Al) and ytterbium (Yb) as a rare earth element, in which aluminum andytterbium are doped so as to satisfy the inequality (A), taking aconcentration of aluminum contained in the core as D_(Al) (unit: % bymass), and a peak absorption coefficient of the absorption band whichappears around a wavelength of 976 nm in the absorption band byytterbium contained in the core as A_(Yb) (unit: dB/m).

In the inequality (A), T_(PD) is an allowable loss increase byphotodarkening at a wavelength of 810 nm in the Yb-doped core opticalfiber, expressed in a unit of dB. The T_(PD) is a value as determinedwhen an optical fiber laser is designed using the Yb-doped core opticalfiber of the present invention, and is a valued determined inconsideration of various factors such as an acceptable value of thedecrease rate of the output power of the optical fiber laser, a useenvironment, an intensity of the pump light source input to the Yb-dopedcore optical fiber, and a desired output power of laser oscillation. IfT_(PD) is set at a certain value, the loss increase by photodarkening ofthe Yb-doped core optical fiber of no more than T_(PD) provides theoptical fiber laser using the Yb-doped core optical fiber with goodcharacteristics. To the contrary, the loss increase by photodarkening ofthe Yb-doped core optical fiber of more than T_(PD) leads tounexpectedly higher decrease in the output power of the laseroscillation in the optical fiber laser using the Yb-doped core, and as aresult, laser oscillation cannot be carried out over a long period oftime.

From the right hand side of the inequality (A), by using two parameters:the concentration of aluminum contained in the core D_(Al) (unit: % bymass) and the peak absorption coefficient of the absorption band whichappears around a wavelength of 976 nm in the absorption band byytterbium contained in the core A_(Yb) (unit: dB/m), the loss increaseby photodarkening of the Yb-doped core optical fiber can be estimated.However, the inequality (A) is an empirical inequality obtained from thedata of the aluminum concentration, the absorption coefficient and theloss increase by photodarkening of a variety of the manufacturedYb-doped core optical fibers. A process for deriving the empiricalinequality will be described later.

As in the present invention, as long as the rare earth-doped coreoptical fiber has the concentration of aluminum contained in the coreand the peak absorption coefficient of the absorption band which appearsaround a wavelength of 976 nm in the absorption band by ytterbiumcontained in the core, which are each adjusted so as to obtain anallowable loss increase by photodarkening, the optical fiber laser usingthe rare earth-doped core optical fiber of the present invention, evenwith the ytterbium concentration varying in the Yb-doped core opticalfiber, has good characteristics. Particularly,

-   -   even when laser oscillation is carried out over a long period of        time, most of the output power of the light at a laser        oscillation wavelength is not decreased, and thus it is capable        of maintaining a sufficient output power of laser oscillation        even with use over a long period of time;    -   even when the ytterbium concentration in the Yb-doped core        optical fiber is high, decrease in the output power of the light        at a laser oscillation wavelength can be maintained small;    -   since the ytterbium concentration in the Yb-doped core optical        fiber can be set high, the length of the fiber required for        laser oscillation may be shorter, and by this, reduction in        cost, inhibition of generation of noise light by a non-linear        optical phenomenon, and the like can be attained; and    -   other effects can be attained.

Third Example of First Embodiment

FIG. 2B is a schematic view showing the third example of the firstembodiment of the rare earth-doped core optical fiber 10C. This rareearth-doped core optical fiber 10C is composed of a core 11C comprisinga silica glass containing Al, Yb as a rare earth element, fluorine (F),and a clad 12C comprising a silica glass provided around the core. Inthe case where Al is doped into the core portion, a higher Alconcentration increases the refractive index of the core, therebycausing change in the optical characteristics such as the mode fielddiameter and the cut-off wavelength. However, in the present example, bydoping fluorine into the core, it becomes possible to dope Al at a highconcentration while maintaining refractive index of core or relativerefractive index difference from the clad in a degree suited for anoptical fiber, by compensating the increase in the refractive indexresulting from the increased Al concentration only.

(Manufacturing Method of Rare Earth-Doped Core Optical Fiber of PresentInvention)

FIGS. 4 and 5 are schematic views each showing one example of themanufacturing method of the rare earth-doped core optical fiberaccording to the present invention in the sequence in the process.

In the manufacturing method of the present invention, first, adeposition step as follows is carried out. A silica glass tube 20 havinga suitable outer diameter and a suitable thickness is first prepared,and as shown in FIG. 4A, as halide gases, SiCl₄ and AlCl₃, and O₂ gasesare transferred from a first cross-section the silica glass tube 20 to ahollow portion of the silica glass tube 20. Then, the silica glass tube20 is heated by an oxyhydrogen burner 22 as a heating means, and SiCl₄and AlCl₃ are oxidized to form a soot-like exit comprising SiO₂ andAl₂O₃, which is deposited on the inner surface of the silica glass tube20. Then, the deposited soot-like exit is sintered, and a porous glasslayer 21 is deposited.

Further, in the deposition step as shown in FIG. 4A, the raw materialgases are SiCl₄, AlCl₃, and O₂ gases, additionally other halide gases,for example GeCl₄ may be appropriately used. If GeCl₄ is introduced,GeO₂ is produced as an exit. Further, for the purpose of lowering therefractive index of the core, SiF₄ may be used in the deposition step.Alternatively, SiF₄ is not used in the deposition step, but it may beonly in the below-described transparentization step (FIG. 5C). Further,fluorine compounds other than SiF₄ (for example, SF₆, CF₄, and C₂F₆) maybe used.

In the deposition step as shown in FIG. 4A, when a soot-like exit isproduced and deposited on the inner surface of the silica glass tube 20,the process is performed while moving the oxyhydrogen burner 22 alongthe long axis of the silica glass tube 20 so as to uniformly deposit theexit on the silica glass tube 20. At this time, it is necessary tocarefully control the heating temperature of the oxyhydrogen burner sothat the deposited soot-like exit is burned and solidified to form aporous glass layer 21. If the temperature is too high, the porous glasslayer 21 becomes a transparent glass, and as a result, the doping stepcannot be carried out. Here, the heating temperature by the oxyhydrogenburner is as low as from a temperature providing a transparent glass toaround 200 to 300° C. for solidifying the exit.

The reciprocation movement of the oxyhydrogen burner 22 is repeatedlycarried out once or several times, to form a porous glass layer 21containing SiO₂ and Al₂O₃.

Next, a doping step as follows is carried out. A raw material gas andthe oxyhydrogen burner 22 are stopped and left to cool. Then, a plug 24is positioned on one side of the silica glass tube 20 having the porousglass layer 21 formed on its inner surface, and the tube is stood withthe plug 24 down side, and as shown in FIG. 4B, injecting an aqueoussolution 23 containing a rare earth element compound from the othercross-section into the tube to penetrate the aqueous solution 23 intothe porous glass layer 21, thereby doping the rare earth element intothe aqueous solution to the porous glass layer 21.

The aqueous solution containing the rare earth element is selectedaccording to the rare earth element doped into the core to the solution.In the manufacturing of the Yb-doped core optical fiber, it ispreferable that the solute of the aqueous solution containing the rareearth element is YbCl₃, and the solvent is H₂O. In this case, the YbCl₃concentration in the aqueous solution 23 is, for example, 0.1 to 5% bymass, and the solution concentration for obtaining a desired Ybconcentration is empirically determined.

The porous glass layer 21 of the inner surface of the silica glass tube20 is immersed in the aqueous solution containing the rare earth elementcompound for a suitable time, such as about 3 hours, and the plug 24 isdetached. Then, the aqueous solution 23 is withdrawn from the tube, andas shown in FIG. 5A, the dried O₂ gas is transported into the silicaglass tube 20 to evaporate the moisture. This drying process is carriedout for 1 hour or longer, preferably about 6 hours.

In order to remove the remaining moisture, while Cl₂, O₂, and He gasesare transported into the silica glass tube 20, the periphery of thesilica glass tube 20 is heated by the oxyhydrogen burner 22 tosufficiently dehydrate the moisture (FIG. 5B). In this case, theoperation is conducted at a heating temperature that is sufficiently lownot to make the porous glass layer 21 transparent.

Thereafter, while SiF₄, He, and O₂ are transported into the silica glasstube 20, and the fire power of the oxyhydrogen burner 22 is raised toperform the process to make the porous glass layer 21 transparent (FIG.5C). In this transparentization step, fluorine can be doped into thetransparentized glass layer (transparent glass layer 25) by flowing SiF₄as a fluorine compound thereinto. Further, as described above, SiF₄ maynot be used in this transparentization step, but it may be used only inthe deposition step. Furthermore, fluorine compounds other than SiF₄(for example, SF₆, CF₄, and C₂F₆) may be used. By using either one,fluorine can be doped into the transparent glass layer 25.

Next, a core solidification step in which the fire power of theoxyhydrogen burner 22 is increased to carry out core solidification ofthe silica glass tube 20, to prepare a rod-like preform 28 is carriedout (FIG. 5D). A core portion 26 containing a silica glass doped withAl, F, and Yb is positioned in the center of the preform 28, whichcorresponds to the core of the optical fiber obtained from the preform28. The clad glass layer 27 formed by core solidification of the silicaglass tube 20 is formed on the periphery of the core portion 26.

The silica tube that is an outer portion of the clad glass layer iscovered on the outside of the prepared preform 28, and a jacket processfor heating integration is carried out to prepare an optical fiberpreform. The preform is drawn to obtain a rare earth-doped core opticalfiber.

Further, a method for forming a clad glass layer is not limited to amethod by the jacket process, and it may be an outside vapor phasedeposition method.

By the above-described manufacturing method, Al can be uniformlycontained in the porous glass layer 21 deposited on the inner surface ofthe silica glass tube 20. The present inventors have found out that whenthe prepared porous glass layer 21 is immersed in the aqueous solution23 containing the rare earth element, the decrease in the output powerof the optical fiber laser is suppressed, as compared to the case whereAl is not contained in the production of a porous glass layer.Particularly, the present inventors have found out that in the casewhere the rare earth element is Yb, the optical fiber laser, constitutedusing the Yb-doped core optical fiber obtained by drawing the preformobtained by the manufacturing method of the present invention, does notdecrease the output power of the light at a laser oscillation wavelengthof 1060 nm, even when laser oscillation is carried out over a longperiod of time. Accordingly, the manufacturing method of the presentinvention, and the Yb-doped core optical fiber obtained by themanufacturing method of the Yb-doped core optical fiber, can be used toobtain an optical fiber laser capable of maintaining sufficient outputpower of laser oscillation even when used over a long period of time.

Second Embodiment

FIG. 6 is a schematic view showing the second embodiment of the rareearth-doped core optical fiber according to the present invention. Therare earth-doped core optical fiber 10D of the present embodiment has aconstitution provided with a polymer layer 13 having a lower refractiveindex than the dads 12B and 12C on the periphery of the dads 12B and 12Cof the rare earth-doped core optical fibers 10B and 10C of theabove-described first embodiment. The core 11D and the clad 12D in therare earth-doped core optical fiber 10D of the present embodiment canhave the same constitution as the cores 11B, 11C, and the dads 12B, 12Cin the rare earth-doped core optical fibers 10B and 10C of theabove-described first embodiment.

By using such a structure, the rare earth-doped core optical fiber ofthe present invention can be a double-clad fiber, and thus by insertinga higher power of the pump light, a higher output power of laseroscillation can be obtained. In a conventional rare earth-doped coreoptical fiber, a higher power of the pump light leads to more remarkabledecrease in the output power of the laser oscillation, and it cannot beused as the double-clad fiber. On the other hand, the rare earth-dopedcore optical fiber of the present invention has a core having the samecomposition as described above, and if it is a double-clad fiber havingthe polymer layer 13 on the periphery of the clad 12D as shown in thepresent embodiment, it is possible to carry out laser oscillation over along period of time.

Third Embodiment

FIG. 7 is a schematic view showing the third embodiment of the rareearth-doped core optical fiber according to the present invention. Therare earth-doped core optical fiber 10E of the present embodiment iscomposed of a core 11E, an inner clad 14 positioned on the exterior ofthe core 11E, an outer clad 15 positioned outside the inner clad 14, anda polymer layer 13 positioned outside the outer clad 15. The core 11Ecomprises a silica glass containing a rare earth element such as Al andYb, and fluorine (F), the inner clad comprises a silica glass containingGe, and the outer clad comprises a silica glass. This rare earth-dopedcore optical fiber 10E has a structure having a refractive indexsatisfying the relationship among the refractive index n1 of the core11E, the refractive index n2 of the inner clad 14, the refractive indexn3 of the outer clad, and the refractive index n4 of the polymer layer13 of: n1>n2>n3>n4. That is, the present structure is a triple-cladstructure comprising the clad composed of the inner clad 14, the outerclad 15, and the polymer layer 13.

By using such a structure, the difference in the refractive indicesbetween the core 11E and the inner clad 14, nA (=n1−n2), can be smallerthan the difference in the refractive indices between the core 11E andthe outer clad 15, nB (=n1−n3). Accordingly, the effective area A_(eff)can be larger of the light at a laser oscillation wavelength of 1060 nm,and thus generation of the noise light by a non-linear opticalphenomenon such as Stimulated Raman Scattering, Stimulated BrillouinScattering, and Four Wave Mixing can be reduced. In order to increasethe effective area A_(eff) by a conventional optical fiber, it isnecessary to decrease the difference in the refractive indices betweenthe core and the clad. Thus, the dopant such as Al and germanium shouldbe reduced, but if the Al concentration is small, the decrease rate ofthe output power in the optical fiber laser is increased. The rareearth-doped core optical fiber 10E of the present embodiment can havethe core 11E doped with a sufficient amount of Al, and the effectivearea A_(eff) can be further increased. Further, the optical fiber laserusing the rare earth-doped core optical fiber 10E of the presentembodiment can have higher performance and higher quality.

In the rare earth-doped core optical fiber according to the presentinvention, even when air holes are provided in a part of the clad, adouble-clad fiber can be obtained, in which laser oscillation is carriedout over a long period of time. Furthermore, by optimization of thepositions of the air holes, a higher NA, a reduced skew light, or thelike can be attained.

EXAMPLES Example 1

Using the Yb-doped core optical fiber having a structure as shown inFIG. 2A, a plurality of optical fibers having Al doped into the coredifferent Yb concentrations were prepared. The clad outer diameter ofthe prepared Yb-doped core optical fiber was 125 μM, the core diameterwas in a range of from 5 to 11 μm according to the Al concentrations,and the Al concentrations in the core were in four classes of 0% bymass, 1% by mass, 2% by mass, and 3% by mass, respectively. Furthermore,a plurality of these Yb-doped core optical fibers having different Ybconcentrations were prepared, and the amount of the peak absorptioncoefficient is varied within a range of from 100 dB/m to 1500 dB/m inthe absorption band which appears around a wavelength of 976 nm causedby the Yb.

Evaluation of the characteristics of decrease in the power of the laseroscillation light of the prepared Yb-doped core optical fiber wasconducted with reference to “Measurement System of Photodarkening” inNon-Patent Document 2. As described above, it is thought that thedecreased in the power of the laser oscillation light is caused from theloss increase by photodarkening. When the pump light at a wavelength of976 nm is entered with a high power onto the Yb-doped core opticalfiber, photodarkening occurs, thereby leading to loss. By measuring theloss at a certain wavelength after the pump light at a wavelength of 976nm was entered for a certain period of time, the magnitude of theincrease in the loss by photodarkening in the optical fiber to bemeasured can be measured, and it is related to the decrease rate of thelight at a laser oscillation wavelength of 1060 nm. Accordingly, thecharacteristics of decrease in the power of laser oscillation light ofthe Yb-doped core optical fiber can be evaluated.

The prepared Yb-doped core optical fiber was set in a measurementinstrument for measuring the loss increase by photodarkening as shown inFIG. 8, and measured. Here, the length of a sample was adjusted under ameasurement condition that the peak absorption coefficient of theoptical fiber to be measured at a wavelength of around 976 nm (unit:dB/m)×the length of the sample (unit: m)=340 dB, and the light power ofthe pump light at a wavelength of 976 nm was set a 400 mW. The lossincrease by photodarkening at a wavelength of 810 nm after entering thepump light for 100 min was measured. The measurement results are shownin FIG. 9A and FIG. 9B.

As shown in FIG. 9, it can be seen that as the absorption coefficientper unit length is higher, that is, as the Yb concentration of theoptical fiber core portion is higher, the loss increase byphotodarkening at a wavelength of 810 nm is higher. Furthermore, as theAl concentration of the optical fiber core portion is higher, the lossincrease by photodarkening at a wavelength of 810 nm is lower.

Next, an optical fiber laser was constituted by using the Yb-doped coreoptical fiber, and subject to laser oscillation over a long period oftime, and then the output power of the light at a laser oscillationwavelength of 1060 nm was observed. The optical fiber laser constitutedby using the Yb-doped core optical fiber having a loss increase byphotodarkening at a wavelength of 810 nm of 0.5 dB or less, the outputpower of the light at a laser oscillation wavelength of 1060 nm was notsubstantially reduced even when laser oscillation was carried out over along period of time. On the other hand, the optical fiber laserconstituted by using the Yb-doped core optical fiber having a lossincrease by photodarkening at a wavelength of 810 nm of more than 0.5dB, the output power of the light at a laser oscillation wavelength of1060 nm was observed to be decreased over time. Furthermore, as the lossincrease by photodarkening was higher, the decrease rate of the outputpower of the light at a laser oscillation wavelength of 1060 nm washigher.

As clearly shown from FIG. 9B, by constituting the Yb-doped core opticalfiber such that the core had an Al concentration of 2% by mass or more,and Yb was contained at such a concentration that a absorption bandwhich appeared around a wavelength of 976 nm showed a peak absorptioncoefficient of 800 dB/m or less in the absorption band by Yb containedin the core, an Yb-doped core optical fiber having a loss increase byphotodarkening at a wavelength of 810 nm of 0.5 dB or less can beobtained.

Example 2

Using the Yb-doped core optical fiber having a structure as shown inFIG. 2B, a plurality of optical fibers having different Ybconcentrations in the core were prepared. The absorption varied withinthe range such that the absorption band which appeared around awavelength of 976 nm showed the peak absorption coefficient in a rangeof from 100 dB/m to 1500 dB/m. The prepared Yb-doped core optical fiberhad a clad outer diameter of 125 μm, a core diameter of approximately 10μm, and an Al concentration in the core of 2% by mass. Furthermore,fluorine (F) was also contained in the core, in addition to Al and Yb.The specific refractive index Δ of the core with respect to the clad wasabout 0.12%.

On the other hand, in Reference Example, using the Yb-doped core opticalfiber having a structure as shown in FIG. 2B, a plurality of opticalfibers having different Yb concentrations in the core were prepared. Theabsorption varied within the range such that the absorption band whichappeared around a wavelength of 976 nm showed the peak absorptioncoefficient in a range of from 100 dB/m to 1500 dB/m. This Yb-doped coreoptical fiber had a clad outer diameter of 125 μm and a core diameter ofapproximately 10 and had no fluorine in the core. It also had an Alconcentration in the core of 1% by mass. The difference Δ in thespecific refractive indices of the core from the clad was about 0.12%.

In a similar manner to Example 1, the loss increase by photodarkening ata wavelength of 810 nm was measured. The results are shown in FIG. 10.As seen from FIG. 10, whether the core contained fluorine or not, havinga higher Al concentration in the optical fiber core portion correspondedto a smaller loss increase by photodarkening at a wavelength of 810 nm.

On the other hand, since the specific refractive index A of the corewith respect to any optical fiber was about 0.12%, the opticalcharacteristics such as the mode field diameter and the cut-offwavelength were the same. Accordingly, by constituting the Yb-doped coreoptical fiber such that the core had an Al concentration of 2% by massor more, Yb was contained at such a concentration that an absorptionband which appeared around a wavelength of 976 nm showed a peakabsorption coefficient of 800 dB/m or less in the absorption band by Ybcontained in the core, and fluorine was contained in the core, anYb-doped core optical fiber having a loss increase by photodarkening ata wavelength of 810 nm of 0.5 dB or less can be obtained, with a smalldifference Δ in the specific refractive indices of the core.

Example 3

According to the manufacturing method of the rare earth-doped coreoptical fiber according to the present invention, a Yb-doped coreoptical fiber was prepared. The prepared optical fiber was a Yb-dopedcore optical fiber having a structure as shown in FIG. 2B, and aplurality of optical fibers having different Yb concentrations in thecore were prepared. The amount of the peak absorption coefficient isvaried within the range of from 100 dB/m to 1500 dB/m in the absorptionband which appears around a wavelength of 976 nm caused by the Ybconcentrations. The prepared Yb-doped core optical fiber had a cladouter diameter of 125 μm, a core diameter of approximately 10 μm, and anAl concentration in the core of 2% by mass. Furthermore, fluorine (F)was also contained in the core, in addition to Al and Yb. The specificrefractive index Δ of the core with respect to the clad was about 0.12%.In the manufacturing of the preform of the present Example 3, Al dopingwas performed in the deposition step as shown in FIG. 4A.

On the other hand, in Reference Example, a Yb-doped core optical fiberwas prepared in the same manner as in Example 3, except that Al dopingfor a preform was performed in the doping step in FIG. 4B. However, inthe present Reference Example, the doping step was performed using anaqueous solution containing AlCl₃ as an Al compound in addition to therare earth element compound, which was different from the doping step asshown in FIG. 4B. The Yb-doped core optical fiber of Reference Example,obtained from the preform prepared by this manufacturing method had thesame Al concentration (2% by mass), and F and Yb concentrations as theYb-doped core optical fiber of Example 3, and the specific refractiveindex of the core was about 0.12%.

In a similar manner to Example 1, the loss increase by photodarkening ata wavelength of 810 nm was measured. The results are shown in FIG. 11.

In any of the optical fibers, the Al concentration in the core was 2% bymass, and the loss increase by photodarkening at a wavelength of 810 nmwas sufficiently small, but there were differences according to themanufacturing methods. The optical fiber Example 3 in which the Aldoping was performed in the deposition step as shown in FIG. 4A had asmaller loss increase by photodarkening than the optical fiber ofReference Example in which the Al doping was performed in the dopingstep as shown in FIG. 4B.

Example 4

Here, the method for deriving the inequality (A) is described.

For the measurement results of the loss increase by photodarkening theYb-doped core optical fiber in Example 1, we tried to express therelationship between the absorption coefficient and the concentration ofaluminum contained in the core by an empirical equation. The data inFIG. 9 was used to determine the empirical equation. Logarithmicexpression of the loss increase at 810 nm of FIG. 9A is shown in FIG.12. The curve in FIG. 12 approximates an exponential function, and canbe expressed by the following empirical equation (1).log(L _(PD))=C ₀ −C ₁*exp{−C ₂*(A _(Yb))}  (1)

wherein L_(PD) is a loss increase by photodarkening at a wavelength of810 nm (unit: dB), and A_(Yb) is an absorption coefficient per unitlength (unit: dB/m). C₀, C₁, and C₂ are fitting factors. For the data ofeach Al concentration in FIG. 12, the empirical equation (1) was usedfor each fitting. For best fit, the fitting factors, C₀, C₁, and C₂ wereadjusted for fitting. For each Al concentration, C₀, C₁, and C₂ weredetermined, as shown in Table 2.

TABLE 2 Fit factor obtained from empirical equation (1) Fit factors Alconcentration C₀ C₁ C₂ Al 0% by mass 1.274 4.352 0.00347 Al 1% by mass0.618 4.256 0.00337 Al 2% by mass −0.037 4.403 0.00353 Al 3% by mass−0.692 4.206 0.00335 Average value 4.304 0.00343

As shown in Table 2, it is found that the fitting factors C₁ and C₂ givealmost the same values even under different Al concentrations, whereasthe fitting factor C₀ varies depending on the Al concentration. Sincethe fitting factors C₁ and C₂ are substantially not changed depending onthe Al concentrations, the average values of C₁ and C₂ obtained fromeach Al concentration, C₁=4.304 and C₂=0.00343 were substituted into theempirical equation (1), thereby obtain the following empirical equation(2).log(L _(PD))=C ₀−4.304*exp{−0.00343*(A _(Yb))}  (2)

It is expected that the fitting factor C₀ is variable depending on theAl concentration.

Next, in the data shown in FIG. 12, the relationship between the lossincreases due to photodarkening at a wavelength of 810 nm and the Alconcentrations was investigated in consideration of the absorptioncoefficient per unit length of 800 dB/m. FIG. 13 shows the relationshipbetween the loss increases at 810 nm and the Al concentrations. FIG. 13has a logarithmic expression of the loss increase at 810 nm. FIG. 13shows the linear relationship between the logarithmic values of the lossincrease at 810 nm and the Al concentrations, thereby it being expressedby the following empirical equation (3).log(L _(PD))=−0.655*(D _(Al))+0.997  (3)

wherein D_(Al) is an Al concentration in the core (unit: % by mass).

Since the empirical equation (3) is an equation derived only from thedata in a case where the absorption coefficient per unit length is 800dB/m, 800 dB/m is substituted into A_(Yb) in the empirical equation (2),thereby obtaining the following equation (4).log(L _(PD))=C ₀−0.277  (4)

By substituting the equation (4) into the equation (3), the followingequation (5) was obtained.C ₀=−0.655*(D _(Al))+1.274  (5)

By substituting the equation (5) into the equation (2), the followingequation (6) was obtained.log(L _(PD))=−0.655*(D _(Al))−4.304*exp{−0.00343*(A _(Yb))}+1.274  (6)

By modifying the equation (6), the following equation (7) was obtained.L _(PD)=10^({−0.655*(D) ^(Al) ^()−4.304*exp{−0.00343*(A) ^(Yb)^()}+1.274})  (7)

Therefore, the equation (7) is an empirical equation showing therelationship between the absorption coefficients and the aluminumconcentrations contained in the core, for the measurement results of theloss increase by photodarkening. If a measured value of the lossincrease by photodarkening, L_(PD), is no more than an allowable lossincrease by photodarkening, T_(PD), as described above, that is, in thecase of the following inequality (8):T _(PD) ≧L _(PD)  (8)the optical fiber laser obtained using this Yb-doped core optical fiberwould have good characteristics.

From the equation (7) and inequality (8), the inequality (A) is derived.T _(PD)≧10^({−0.655*(D) ^(Al) ^()−4.304*exp{−0.00343*(A) ^(Yb)^()}+1.274})  (A)

As shown in Examples 1 and 2, the optical fiber laser constituted byusing the Yb-doped core optical fiber having a loss increase byphotodarkening of 0.5 dB or less, the output power of the light at alaser oscillation wavelength of 1060 nm was substantially reduced, evenwhen laser oscillation was carried out over a long period of time. Inorder to obtain such the Yb-doped core optical fiber, by using theinequality (B) obtained by setting an allowable loss increase byphotodarkening in the inequality (A) to T_(PD)=0.5 dB, the absorptioncoefficients and the aluminum concentrations should satisfy therelationship in this inequality.0.5≧10^({−0.655*(D) ^(Al) ^()−4.304*exp{−0.00343*(A) ^(Yb)^()}+1.274})  (B)

To confirm the effect of the inequality (B), using the Yb-doped coreoptical fiber having a structure as shown in FIG. 2A, 9 kinds of fibershaving different Al concentrations in the core and absorptioncoefficients were prepared. The Al concentration and the absorptioncoefficient of each fiber are shown in Table 3.

TABLE 3 List of Yb-doped core optical fibers prepared Yb light Empiricalabsorption Al equation (8) Measured loss Kind of Optical rateconcentration satisfied or increment at fiber fiber (dB/m) (% by mass)not 810 nm (dB) Fiber of Sample A 600 1.11 X 1.00 Comparative ExampleFiber of the Sample B 600 1.57 ◯ 0.50 present invention Fiber of theSample C 600 1.9 ◯ 0.30 present invention Fiber of Sample D 800 1.52 X1.00 Comparative Example Fiber of the Sample E 800 1.98 ◯ 0.50 presentinvention Fiber of the Sample F 800 2.32 ◯ 0.30 present invention Fiberof Sample G 1000 1.73 X 1.00 Comparative Example Fiber of the Sample H1000 2.19 ◯ 0.50 present invention Fiber of the Sample I 1000 2.53 ◯0.30 present invention

For the samples, A, B, and C, the absorption coefficients are all 600dB/m, but the Al concentrations are different from each other. For thesamples, D, E, and F, the absorption coefficients are all 800 dB/m, butthe Al concentrations are different from each other. For the samples, G,H, and I, the absorption coefficients are all 1000 dB/m, but the Alconcentrations are different from each other. In Comparative Examples,if the absorption coefficient and the Al concentration of each of thesamples, A, D, G are substituted into the inequality (B), the right handside of the inequality (B) is more than 0.5 in any of the fibers, andaccordingly it does not satisfy the condition of the inequality (B). Onthe other hand, the samples, B, C, E, F, H, and I, that are the opticalfibers of the present invention, all satisfy the condition of theinequality (B).

In a similar manner to Example 1, for the Yb-doped core optical fibersof the samples A through I, the loss increase by photodarkening at awavelength of 810 nm was measured. The results are shown in Table 3. Asseen from Table 3, the samples, B, C, E, F, H, and I, that are theoptical fibers of the present invention, all have a loss increase byphotodarkening of 0.5 dB or less. On the other hand, the samples, A, D,and G in Comparative Examples, all had a loss increase by photodarkeningof more than 0.5 dB.

As clearly seen from Table 3, by doping aluminum and ytterbium in thecore such that the concentration of aluminum contained in the core, andthe peak absorption coefficient of the absorption band which appears ata wavelength of 976 nm in the absorption band by ytterbium contained inthe core satisfy the inequality (B), it is possible to obtain anYb-doped core optical fiber having a loss increase by photodarkening ata wavelength of 810 nm of 0.5 dB or less.

1. A rare earth-doped core optical fiber comprising: a core whichcomprises a silica glass containing at least aluminum and ytterbium; anda clad provided around the core and comprising a silica glass having alower refraction index than that of the core, wherein aluminum andytterbium are doped into the core such that a loss increase byphotodarkening, T_(PD), satisfies the following inequality (A):T _(PD)≧10^({−0.655*(D) ^(Al) ^()−4.304*exp{−0.00343*(A) ^(Yb)^()}+1.274})  (A) wherein T_(PD) represents an allowable loss increaseby photodarkening at a wavelength of 810 nm (unit: dB), D_(Al)represents the concentration of aluminum contained in the core (unit: %by mass), and A_(Yb) represents the peak absorption coefficient of theabsorption band which appears around a wavelength of 976 nm in theabsorption band by ytterbium contained in the core (unit: dB/m), whereinT_(PD) is 0.5 dB or less, and wherein the core includes no other dopantthat increases the refractive index except for rare earth dopants. 2.The rare earth-doped core optical fiber according to claim 1, whereinthe core has an aluminum concentration of 2% by mass or more, andytterbium is doped into the core at such a concentration that theabsorption band of ytterbium doped into the core which appears around awavelength of 976 nm shows a peak absorption coefficient of 800 dB/m orless.
 3. The rare earth-doped core optical fiber according to claim 2,wherein the core further contains fluorine.
 4. The rare earth-doped coreoptical fiber according to claim 1, wherein a polymer layer having alower refraction index than that of the clad is provided on the outerperiphery of the clad.
 5. The rare earth-doped core optical fiberaccording to claim 4, wherein the clad is composed of an inner cladpositioned on the exterior of the core, and an outer clad positionedoutside the inner clad, and the refractive index n1 of the core, therefractive index n2 of the inner clad, the refractive index n3 of theouter clad, and the refractive index n4 of the polymer layer satisfy therelationship of n1>n2>n3>n4.
 6. The rare earth-doped core optical fiberaccording to claim 1, wherein air holes are present in a part of theclad glass.
 7. A rare earth-doped core optical fiber comprising: a corewhich comprises a silica glass containing at least aluminum, ytterbium,and fluorine; and a clad provided around the core and comprising asilica glass having a lower refraction index than that of the core,wherein aluminum and ytterbium are doped into the core such that a lossincrease by photodarkening, T_(PD), satisfies the following inequality(A):T _(PD)≧10^({−0.655*(D) ^(Al) ^()−4.304*exp{−0.00343*(A) ^(Yb)^()}+1.274})  (A) wherein T_(PD) represents an allowable loss increaseby photodarkening at a wavelength of 810 nm (unit: dB), D_(Al)represents the concentration of aluminum contained in the core (unit: %by mass), and A_(Yb), represents the peak absorption coefficient of theabsorption band which appears around a wavelength of 976 nm in theabsorption band by ytterbium contained in the core (unit: dB/m), whereinT_(PD) is 0.5 dB or less, and wherein the core includes no other dopantthat increases the refractive index except for rare earth dopants.